Two-cycle motors deliver one power impulse for each revolution of the crankshaft.

Two-cycle engines can be found nearly everywhere these days. They are used in dozens of applications and
in a wide variety of designs for everything from work and recreation to power generation. Two-cycle engines have design
differences and operate under conditions that require different oil chemistries than their four-cycle counterparts.
In order to recommend a lubricant for a two-cycle engine, one needs to know how this engine operates, why it is used in
place of a four-cycle engine and where and in what type of applications it is used.

What is a two-cycle engine?

The terms "two-cycle" and "two-stroke" are often inter-changed when speaking about
two-cycle engines. These engines derive their name from the amount of directional changes that the pistons make during each
power stroke. Internal combustion engines are used to produce mechanical power from the chemical energy contained in hydrocarbon
fuels. The power-producing part of the motor's operating cycle starts inside the motor's cylinders with a compression process.
Following this compression, the burning of the fuel-air mixture then releases the fuel's chemical energy and produces
high-temperature, high-pressure combustion products. These gases then expand within each cylinder and transfer work to the piston.
Thus, as the engine is operated continuously, mechanical power is produced. Each upward or downward movement of the piston is called
a stroke. There are two commonly used internal combustion engine cycles: the two-stroke cycle and the four-stroke cycle.

A four-cycle engine requires four strokes of the piston
(two up and two down) and two revolutions of the crankshaft to
complete one combustion cycle and provide one power impulse.

How are two-cycle engines
different from four-cycle engines?

The fundamental difference
between two-cycle engines and four-cycle engines is in their gas exchange
process, or more simply, the removal of the burned gases at the end of
each expansion process and the induction of a fresh mixture for the next
cycle. The two-cycle engine has an expansion, or power stroke, in each
cylinder during each revolution of the crankshaft. The exhaust and the
charging processes occur simultaneously as the piston moves through its
lowest or bottom center position.

In a four-cycle engine, the
burned gasses are first displaced by the piston during an upward stroke,
and then a fresh charge enters the cylinder during the following downward
stroke. This means that four-cycle engines require two complete turns of
the crankshaft to make a power stroke, versus the single turn necessary in
a two-cycle engine. In other words, two-cycle engines operate on 360
degrees of crankshaft rotation, whereas four-cycle engines operate on 720
degrees of crankshaft rotation.

Where are two-cycle engines used?

Two-cycle engines are inexpensive to build and operate when compared to
four-cycle engines. They are lighter in weight and they can also produce a
higher power-to-weight ratio. For these reasons, two-cycle engines are
very useful in applications such as chainsaws, Weed-eaters, outboards,
lawnmowers and motorcycles, to name just a few. Two-cycle engines are also
easier to start in cold temperatures. Part of this may be due to their
design and the lack of an oil sump. This is a reason why these engines are
also commonly used in snowmobiles and snow blowers.

Some advantages and disadvantages of two-cycle engines

Because two-cycle engines can effectively double the number of power
strokes per unit time when compared to four-cycle engines, power output is
increased. However, it does not increase by a factor of two. The outputs
of two-cycle engines range from only 20 to 60 percent above those of
equivalent-size four-cycle units. This lower than expected increase is a
result of the poorer than ideal charging efficiency, or in other words,
incomplete filling of the cylinder volume with fresh fuel and air. There
is also a major disadvantage in this power transfer scenario. The higher
frequency of combustion events in the two-cycle engine results in higher
average heat transfer rates from the hot burned gases to the motor's
combustion chamber walls. Higher temperatures and higher thermal stresses
in the cylinder head (especially on the piston crown) result. Traditional
two-cycle engines are also not highly efficient because a scavenging
effect allows up to 30 percent of the unburned fuel/oil mixture into the
exhaust. In addition, a portion of the exhaust gas remains in the
combustion chamber during the cycle. These inefficiencies contribute to
the power loss when compared to four-cycle engines and explains why
two-cycle engines can achieve only up to 60 percent more power.

How are two-cycle engines lubricated?

Two-cycle motors are considered total-loss type lubricating systems.
Because the crankcase is part of the intake process, it cannot act as an
oil sump as is found on four-cycle engines. Lubricating traditional
two-cycle engines is done by mixing the oil with the fuel. The oil is
burned upon combustion of the air/fuel mixture. Direct Injection engines
are different because the fuel is directly injected into the combustion
chamber while the oil is injected directly into the crankcase. This
process is efficient because the fuel is injected after the exhaust port
closes, and therefore more complete combustion of fuel occurs and more
power is developed. Direct injection engines have a higher power density
than traditional two-cycle engines. Because the oil is directly injected
into the crankcase, less oil is necessary and lower oil consumption
results (80:1 range). Direct Injection motors have higher combustion
temperatures, often up to 120°F. They also require more lubricity than
traditional two-cycle motors.

Two-cycle engines remain the workhorses in
recreational equipment, including outboard motors, motorcycles,
snowmobiles and personal watercraft, as well as in lawn and garden
applications like lawn mowers and weed eaters. Design changes over recent
years have included direct fuel injection engines, in which fuel is
injected directly into the combustion chamber, and exhaust power valves,
which improve combustion efficiency at varying RPM. These design changes
have allowed two-cycle engine manufacturers to effectively reduce
emissions, improve fuel efficiency and decrease oil consumption. Today's
two-cycle motors of all types require specialized oil technology to
deliver maximum engine protection,
dependable operation and long service life

As the leader in two-cycle oil technology,
AMSOIL has performed extensive research in developing a new line of
revolutionary, specialized oils called the "Fabulous Four."
AMSOIL INTERCEPTOR Synthetic 2-Cycle Oil (AIT) was the first new oil to be
introduced. It is primarily recommended as an injector oil or at a 50:1
mix ratio in carbureted, electronic fuel injected (EFI) and direct fuel
injected (DFI) snowmobiles, personal watercraft, motorcycles and ATV's.
Formulated with wax-free premium quality synthetic esters and a
specialized additive package, INTERCEPTOR is designed to address the
exhaust valve sticking problems associated with certain snowmobile models.

Even though AMSOIL 2-Cycle Oils have been optimized for
specific applications, they are multi-functional and recommended for use
in many areas. The chart rates each oil's performance abilities per
application as follows:

Excellent: "The" primary recommendation
for the oil. This is the best possible choice for the application. The oil
is specifically designed for the operating conditions of the motors in
these markets. There is only one "Excellent" recommendation per
category.

Very Good: A main recommendation for the oil.
Identifies an oil that is very good for the operating conditions of these
motors.

Good: A secondary recommendation for an oil. The
oil was not specifically designed for these applications, however, the
chemistry in the oil is suited to the operating conditions of these
motors.

Not Recommended: Identifies applications where
the product was not intended for use. "Excellent" rated products
offer superior performance.

Racing:

(1) Not recommended for long-term use in outboard
motors as a TC-W3 type oil, although excellent as a race oil for short
term use where motors are periodically inspected.

(2) Excellent for racing or modified motors. Very good
for recreational use.